# std::malloc

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< cpp‎ | memory‎ | c

Defined in header `<cstdlib>` |  |
void* malloc( std::size_t size ); |  |

Allocates size bytes of uninitialized storage.

If allocation succeeds, returns a pointer to the lowest (first) byte in the allocated memory block that is suitably aligned for any scalar type (at least as strictly as std::max_align_t) (implicitly creating objects in the destination area).

If size is zero, the behavior is implementation defined (null pointer may be returned, or some non-null pointer may be returned that may not be used to access storage, but has to be passed to std::free).

The following functions are required to be thread-safe: The library versions of `operator new` and `operator delete` User replacement versions of global `operator new` and `operator delete` std::calloc, std::malloc, std::realloc, std::aligned_alloc(since C++17), std::free Calls to these functions that allocate or deallocate a particular unit of storage occur in a single total order, and each such deallocation call happens-before the next allocation (if any) in this order. | (since C++11)

### Parameters

size | - | number of bytes to allocate

### Return value

On success, returns the pointer to the beginning of newly allocated memory. To avoid a memory leak, the returned pointer must be deallocated with std::free() or std::realloc().

On failure, returns a null pointer.

### Notes

This function does not call constructors or initialize memory in any way. There are no ready-to-use smart pointers that could guarantee that the matching deallocation function is called. The preferred method of memory allocation in C++ is using RAII-ready functions std::make_unique, std::make_shared, container constructors, etc, and, in low-level library code, new-expression.

For loading a large file, file mapping via OS-specific functions, e.g. `mmap` on POSIX or `CreateFileMapping`(`A`/`W`) along with `MapViewOfFile` on Windows, is preferable to allocating a buffer for file reading.

### Example

Run this code

    #include <cstdlib>
    #include <iostream>
    #include <memory>
    #include <string>

    int main()
    {
        constexpr std::size_t size = 4;
        if (auto ptr = reinterpret_cast<std::string*>(std::malloc(size * sizeof(std::string))))
        {
            try
            {
                for (std::size_t i = 0; i < size; ++i)
                    std::construct_at(ptr + i, 5, 'a' + i);
                for (std::size_t i = 0; i < size; ++i)
                    std::cout << "ptr[" << i << "] == " << ptr[i] << '\n';
                std::destroy_n(ptr, size);
            }
            catch (...) {}
            std::free(ptr);
        }
    }

Output:

    p[0] == aaaaa
    p[1] == bbbbb
    p[2] == ccccc
    p[3] == ddddd

### See also

operator newoperator new[] | allocation functions (function)
get_temporary_buffer (deprecated in C++17)(removed in C++20) | obtains uninitialized storage (function template)
C documentation for malloc

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